This invention relates to a method of forming a mixture or premix which can be converted into a dope suitable for producing cellulosic products and which comprises cellulosic material dispersed in a solvent, particularly an amine oxide, e.g. a tertiary amine N-oxide.
McCorsley U.S. Pat. No. 4,416,698, the contents of which are incorporated herein by way of reference, describes a method of producing cellulose filaments involving spinning a hot, viscous solution of cellulose dissolved in a solvent of tertiary amine N-oxide. Such a solution is commonly referred to as a dope. In the preparation of the dope it is stated that cellulose dissolves rapidly and forms a solution of cellulose in tertiary amine N-oxide of more uniform composition if both the tertiary amine N-oxide containing the preferred amount of water and the cellulose are ground to the same predetermined particle size and are then charged simultaneously to the barrel of an extruder where they are heated to an elevated temperature. Typically the cellulose and tertiary amine N-oxide are ground in a mill through a 0.5 mm screen, the cellulose particle size being reduced without significant degradation of the cellulose molecular weight. In the extruder the mixture is heated to dissolve the cellulose in the tertiary amine N-oxide-water mixture to form a dope prior to extrusion to form a filament or film. The McCorsley US patent does not describe in any great detail how the cellulose should be dissolved in the amine oxide solution to provide a high quality dope in which virtually all the cellulose is dissolved in the amine oxide.
The preparation of dope comprising a solution of cellulose in amine oxide is also disclosed in McCorsley et al U.S. Pat. No. 4,211,574, McCorsley et al U.S. Pat. No. 4,142,913 and McCorsley U.S. Pat. No. 4,144,080. In all these patents a cellulose containing mixture is prepared by swelling cellulose fibers in a non-solvent for cellulose, e.g. water, and mixing the cellulose fibers with amine oxide. The resultant product is cooled to ambient temperature to form a solid product of cellulose dispersed in amine oxide. The product is comminuted to form chips which are fed to an extruder. The chips are heated in the extruder causing the cellulose to dissolve in and form a solution with the amine oxide. The cellulose dope so formed can be extruded and for shaping into a cellulose article.
An object of the present invention is to provide a method of forming a high quality premix suitable for producing a cellulosic dope which can be shaped to produce cellulosic products.
According to another object of the present invention, there is described a method of forming a cellulose containing premix by stirring constituents of the premix in a mixing chamber of a mixing device.
According to the present invention, in a method of forming a cellulosic premix having cellulosic material dispersed in a solvent of amine oxide and involving a mixing device having chamber walls defining a horizontal cylindrical mixing chamber and mixing means comprising a rotatable horizontal shaft axially disposed within the cylindrical chamber and having axially spaced apart stirring elements fixed thereto and extending outwardly therefrom, the method comprising introducing into the mixing chamber predetermined quantities of shredded cellulosic material and a solution of amine oxide at an elevated temperature of from 60.degree. to 80.degree. C. and rotating the shaft at a speed of between 40 and 80 r.p.m. for a period of time.
Conveniently the amine oxide comprises any suitable tertiary amine oxide which is compatible with water. Preferred tertiary amine oxides are cyclic mono (N-methylamine-N-oxide) compounds such as, for example, N-methyl-morpholine-N-oxide, N-methylpiperidine-N-oxide, N-methylpyrrolidone-oxide, dimethylcyclohexylamine oxide and the like.
Conveniently the axially spaced apart stirring elements comprise plough blades.
Preferably the mixing process also involves rotating refiner blades, mounted in the chamber walls of the mixer, at a relatively high speed in excess of 1,500 r.p.m.. Conveniently a plurality of such refiner blades are mounted in the lower half of the chamber walls defining the mixing chamber and are rotated at speeds in excess of 2,500 r.p.m, e.g. 3000 r.p.m.. The use of refiner blades rotatable about axes orthogonal to the axis of the shaft of the mixing means, produces an efficient form of mixing for mixing the cellulosic material with the amine oxide. It has been found that by using such a mixing device, a high quality premix is produced in which virtually all the cellulosic products are dispersed in the amine oxide solution.
Preferably the shaft of the mixing means is rotated at a speed of between 60 and 80 r.p.m. e.g. 72 r.p.m.
Preferably the contents of the mixing chamber are maintained at an elevated temperature in excess of 150.degree. F., e.g. 176.degree. F. This is achieved by controlling the temperature of the amine oxide solution prior to introduction into the mixing chamber to a desired temperature. Furthermore, the walls of the mixing chamber may be heated, e.g. by circulating heated water therearound, to maintain the temperature of the premix at the elevated temperature.
Suitably, the cellulosic material and amine oxide solution are mixed for a period of at least four minutes, e.g. 6-8 minutes, after introduction of all the constituents to the mixer.
After mixing of the cellulosic material with the amine oxide, the mixed, homogeneous premix is conveniently dispensed into a storage hopper for maintaining the premix in a homogeneous form.